New supernova analysis reframes dark energy debate

The difference in the magnitudes of supernovae in the
ΛCDM and Timescape cosmologies and the magnitudes the
supernovae would appear to have in an empty universe
(horizontal dashed line). Both models show recent apparent
acceleration following earlier deceleration. In the Timescape
model this is not a real effect, however, and the curve is
flatter than the ΛCDM case. Credit: Lawrence Dam, Asta Heinesen
and David Wiltshire

The accelerating expansion of the Universe may not be real,
but could just be an apparent effect, according to new
research published in the journal Monthly Notices of the
Royal Astronomical Society. The new study—by a group at
the University of Canterbury in Christchurch, New
Zealand—finds the fit of Type Ia supernovae to a model
universe with no dark energy to be very slightly better than
the fit to the standard dark energy model.

Dark energy is usually assumed to form roughly 70% of
the present material content of the Universe. However, this
mysterious quantity is essentially a place-holder for unknown
physics.

Current models of the Universe require this dark energy term to explain the observed
acceleration in the rate at which the Universe is expanding.
Scientists base this conclusion on measurements of the
distances to supernova explosions in distant galaxies, which
appear to be farther away than they should be if the Universe’sexpansion were not accelerating.

However, just how statistically significant this signature of
cosmic acceleration is has been hotly debated in the past year.
The previous debate pitted the standard Lambda Cold Dark Matter
(ΛCDM) cosmology against an empty universe whose expansion
neither accelerates nor decelerates. Both of these models
though assume a simplified 100 year old cosmic expansion
law—Friedmann’s equation.

Friedmann’s equation assumes an expansion identical to that of
a featureless soup, with no complicating structure. However,
the present Universe actually contains a complex cosmic web of
galaxy clusters in sheets and filaments that surround and
thread vast empty voids.

This is a computer-simulated image depicting one possible
scenario of how light sources are distributed in the cosmic web.
Credit: Andrew Pontzen and Fabio Governato / Wikimedia Commons
(CC BY 2.0)

Prof David Wiltshire, who led the study from the University of
Canterbury in Christchurch, said, “The past debate missed an
essential point; if dark energy does not exist then a likely
alternative is that the average expansion law does not follow
Friedmann’s equation.”

Rather than comparing the standard ΛCDM cosmological model with an empty universe, the new study compares the fit of
supernova data in ΛCDM to a different model, called the
‘timescape cosmology’. This has no dark energy. Instead, clocks
carried by observers in galaxies differ from the clock that
best describes average expansion once the lumpiness of
structure in the Universe becomes significant. Whether or not
one infers accelerating expansion then depends crucially on the
clock used.

The timescape cosmology was found to give a slightly better fit
to the largest supernova data catalogue than the ΛCDM
cosmology. Unfortunately the statistical evidence is not yet
strong enough to rule definitively in favour of one model or
the other, but future missions such as the European Space
Agency’s Euclid satellite will have the power to distinguish
between the standard cosmology and other models, and help
scientists to decide whether dark energy is real or not.

Deciding that not only requires more data, but also better
understanding properties of supernovae which currently limit
the precision with which they can be used to measure distances.
On that score, the new study shows significant unexpected
effects which are missed if only one expansion law is applied.
Consequently, even as a toy model the timescape cosmology provides a powerful tool to test our
current understanding, and casts new light on our most profound
cosmic questions.